Magnetic deflection system providing plural exit ports for a beam of charged particles



June 2, 1964 s. E. HUNT 3,135,863

MAGNETIC DEFLECTION SYSTEM PROVIDING PLURAL EXIT PORTS FOR A BEAM OF CHARGED PARTICLES 2 Sheets-Sheet l Filed Feb. 7, 1962 June 2, 1964 s. E. HUNT MAGNETIC DEF'LECTION SYSTEM PROVIDING PLURAL EXIT PORTS FOR A BEAM OF CHARGED PARTICLES Filed Feb. 7, 1962 2 Sheets-Sheet 2 a/ il #VVF/V705 574 fr WA/57 HUA/ www m70* @T7-@mvg United States Patent O 3,135,863 MAGNETIC DEFLECTIN SYSTEM PROVIDING PLURAL EXIT PORTS FOR A BEAM F CHARGED PARTICLES Stanley Ernest Hunt, Reading, England, assignor to Associated Electrical Industries Limited, London, England, a British company Filed Feb. 7, 1962, Ser. No. 171,673 Claims priority, application Great Britain Feb. 9, 1961 3 Claims. (Cl. Z50-49.5)

This invention relates to deection systems'for deflecting charged particles.

' Many accelerators of charged particles (for example Van de Graafr accelerators) are more easily built in a manner which produces a vertical beam of charged particles rather than a horizontal beam, while it is usually more convenient to utilise horizontal beams. There is thus a requirement for a dedection system capable of deilecting the accelerated charged particles through a right angle. One method of producing such a deection is by means of a magnetic eld and hitherto such magnetic deectors have a single magnet. It is impossible with a single magnetic detlector to ensure that the beam emerging from the deector is completely parallel sided.

According to the present invention a deflection system for charged particles comprises two or more magnetic deectors spaced apart from one another and each deflecting the particles in succession so that the total angle of dellection is the sum of the angles through which the particles are deected in each deector.

With such an arrangement the distance between the two deectors can be adjustable to alter the focal length of the system and produce a completely parallel sided emergent beam if required.

In order that the invention may be more fully understood, reference will now be made to the accompanying drawings, FIGURE 1 of which illustrates the invention diagrammatically.

Referring to FIG. l, it is required to deflect a downwardly extending incident beam 1 through 90 so as to emerge along a horizontal path 2. To achieve this detlection, two magnetic deilectors illustrated diagrammatically at 3 and 4 are provided, each dellector detlecting the beam through 45. The entry and emergent faces of each deector are arranged to be normal to the beam. If r is the radius of curvature of the beam through the deectors, then the two deflectors 3 and 4 are separated by a distance which is adjustable about the value 2r.

With such an arrangement, the beam deected in the first deiiector 3 becomes focused at a point between the deectors determined by the momentum of the charged particles and thereafter passes through the second deector 4 to emerge along path 2. The exact configuration of the beam along path 2, namely, whether it is convergent, parallel sided or divergent, depends on the distance between the two deectors 3 and 4. In the case of an initially parallel incident beam, if the separation is 2r, then a completely parallel sided beam is emitted, if the separation is greater than 2r a convergent beam is emitted, while if the separation is less than 2r a divergent beam is emitted. It should be noted that since the beam is normal to all pole edges, no defocusing takes place in the horizontal plane.

A further advantage of a deector system according to the invention is that stabilising and control arrangements can be limited to the rst of the 45 deectors only and accordingly will be much less cumbersome and thus much cheaper than where these stabilising or control arrangements are applied to a single deecting magnet which dedects through 90.

One example of this is in the provision of a stabilising voltage to control the accelerator which produces beam 1. It will be appreciated that particles emitted by an accelerator are usually of mixed mass, for example, if hydrogen is used as the ion source, the beam will consist of protons, single ionised hydrogen molecules of mass 2 and composite particles of mass 3 consisting of a proton bound to a hydrogen molecule. These particles of different masses are separated by the magnetic field of a deflector since particles of the same charge which have been accelerated through the same potential diterence have radii of deection inversely proportional to the square root of their masses.

In order to prevent attenuation of the beam of particles of wanted mass, say the protons, one or other ot the heavier particles can be used to provide the correcting signals to stabilise the voltage of the accelerator.

In the case of the deilector system illustrated, the heavier particles can be made to come to a focus in the space between the two magnets. Thus, it point S is the cross-over point of particles of mass 1, particles of mass 2 will focus at a point 6 and particles of mass 3 at a point 7. A suitable exit slit system can be positioned at point 6 or point 7 and the signal'therefrom used to stabilise the voltage from the accelerator.

It will be realised that detlector 3 needs to be enlarged to provide focussing of the additional particles but the degree of enlargement of the dellector is much less than the enlargement required to produce the same effect with a dellector of Thus, there is an overall reduction in cost and weight.

It desired, the iield of the second deflector 4 can be stabilised and measured relatively crudely if required. Alternatively, correcting signals can be fed into the current windings of deilector 4 to move the beam slightly relative to a remote target while keeping the energy of the beam accurately stabilised as defined by a stabilised magnetic eld in the first deilector 3. Another possibility is to slightly vary the magnetic eld of deilector 4 to keep the beam focused on a target irrespective of slight energy iluctuations of the beam allowed by the resolution or stability of the eld of deilector 3.

Apparatus in accordance with one embodiment of the invention is illustrated in FIGS. 2 and 3, FIG. 2 being a side elevation, and FIG. 3 a front elevation.

A source of positively charged particles is indicated generally at 10 and an accelerating tube having a plurality of electrodes is shown at 11. The particles pass axially along the tube and a graded potential applied to the electrodes causes the particles to be accelerated, and they enter the deection system in the form of a beam through a tube 12. The particles enter the system travelling in a vertical plane and, by means of two magnetic dellectors, the particles are deflected through a right angle and leave the system travelling in a horizontal plane. The particles are utilised at a target which is mountable on a target support plate 13.

The deflection system includes two similar electromagnetic dellectors 14 and 14 spaced apart and each arranged to deflect the particles through 45. Each 'deflector comprises a pair of shaped pole pieces 15 spaced apart to provide a slit 16, the pole pieces forming part of a magnetic circuit which is capable of carrying a high value of magnetic flux produced in two coils 17. The deilectors are mounted on a support frame 18 and their position relative to the frame and to each other may be adjusted by means of a plurality of adjusting bolts 19. The deilector 14 is arranged on the mounting frame with the slit 16 normal to the aXis of the tube 12, and similarly the edge of the slit 16, in the deector 14', adjacent the target is arranged to lie in a vertical plane. If a parallel sided beam is required at the target, the deilectors 14 and 14 are spaced apart such that the distance between the output edge of the slit in the deector 14 and the inputY edge of the slit in deilector 14', is equal to twice the radius of curvature of the beam through each deectcr.

'A Y The particles entering/the slit formed in the first deilector are of mixed mass, and they are separated by the magnetic field of they deector sincev particles of the same charge'which have been accelerated through the same potentialdiierence have radii of'dellection inversely proportional to the square root of their mass. YThe lightest particles leaving the deector 14 enter arbeam tube 20 which connects the'slits E6 in the two deectors, These Y particles are further deflected by the second deector and finally reach the target. Particles of heavier mass, however, leave the deector 14 through one or more of the beam tubes 22 as explained Vhereabove relative to the 4focusing of particles at points 6 or Each beam tube 22 is terminated by atarget holder23. The particles,

arriving at the target may be utilised-to produce an electrical signal which is fed to the Vaccelerating tube and used n to stabilise the voltages applied to the accelerating elec-- trodesr. t Y

VWhat I claim is: Y

l. AV deflection system'or a beam of charged particles comprising an evacuatable vessel, a iirst port in said vessel through which said beam enters therein, a second port Y in said vessel through which particles of a given mass and energy egress therefrom, and a path for said particles of Vgiven mass andenergy between said ports, two similar magnetic deectors spaced apart along `said path, eachV deiector in succession serving to deiiect said particles of given mass and energy through 459 around curved paths of radius r, a mounting frame lfor supporting said vessel and said deectors with said deectors separated by a distance substantially equal to 2r and means for moving said deflectors relative to said mounting frame by which the distance'between the deflectors may be adjusted.

2. Adeiiection system for a beam of charged particles comprisingan evacuatable vessel, a first port in said vessel through which said beam enters therein, a second port in said vessel through which particles of a t given mass andV energy egress therefrom, a further port in said vessel and a path for said particles of givenrmass and energy between said ports, two similar magnetic deflectors spaced f apart along said path, each deector insuccession serving to deflect said particles of given mass and energy through around curved pathsy of radius r, said further portV means for moving said d'eectors relative to saidmountf ing frame by which the distance between the deflector may be adjusted. Y v Y 3. The combination with a deiiection vsystem as claimed `in'claim V2 ofY means -for producing charged particles,

meansfor forming said charged particles into a beam, a target for utilizing said particles of a given mass and energy which egress from said vesseland means for sup-Y porting said target.Y

Referenees Cited `in the file of this-patent i i j UNITED sri-irasPATENTSv v2,470,745 schiesman Mayu, 19219 2,544,718 Nier Mar. 13, 1951 2,824,987 Neissenberg et al Feb. 25, 1958` White Aug.V 23g V1960 

1. A DEFLECTION SYSTEM FOR A BEAM OF CHARGED PARTICLES COMPRISING AN EVACUATABLE VESSEL, A FIRST PORT IN SAID VESSEL THROUGH WHICH SAID BEAM ENTERS THEREIN, A SECOND PORT IN SAID VESSEL THROUGH WHICH PARTICLES OF A GIVEN MASS AND ENERGY EGRESS THEREFROM, AND A PATH FOR SAID PARTICLES OF GIVEN MASS AND ENERGY BETWEEN SAID PORTS, TOW SIMILAR MAGNETIC DEFLECTORS SPACED APART ALONG SAID PATH, EACH DEFLECTOR IN SUCCESSION SERVING TO DEFLECT SAID PARTICLES OF GIVEN MASS AND ENERGY THROUGH 45* AROUND CURVED PATHS OF RADIUS R, A MOUNTING FRAME FOR SUPPORTING SAID VESSEL AND SAID DEFLECTORS WITH SAID DEFLECTORS SEPARATED BY A DISTANCE SUBSTANTIALLY EQUAL TO 2R AND MEANS FOR MOVING SAID DEFLECTORS RELATIVE TO SAID MOUNTING FRAME BY WHICH THE DISTANCE BETWEEN THE DEFLECTORS MAY BE ADJUSTED. 